Thyroid hormones

Thyroid hormones are divided into two different classes: yoditironiny (thyroxine, triiodothyronine) and calcitonin. Of the two classes of thyroid hormones thyroxine expert and triiodothyronine regulate the body's basal metabolic rate (the level of energy that is needed to maintain the body's ability to live in a state of complete rest), and calcitonin participates in the regulation of calcium metabolism and bone development

Thyroid hormones are produced in spherical formations, called follicles. follicle cells (so-called A-thyroid cells) produce thyroxine (T4) and triiodothyronine (T3), which are major thyroid hormones. These thyroid hormones are chemically very similar - they only vary by the number of iodine atoms per molecule. Thyroxine molecule contains four atoms of iodine and triiodothyronine molecule - 3 atoms (and hence reduce - T3 and T4). Thyroid hormones are found in the blood in a free form, and in connection with specific carrier proteins. Activity display only available form of thyroid hormone (their abbreviated FT4 and FT3, from the free T4 - free fraction of the hormone T4).

For the synthesis of the two major hormones the thyroid gland needs iodine and the amino acid tyrosine. In synthesizing primarily formed specific protein - thyroglobulin, which accumulates in the thyroid follicular cavity and serves as a "safety" for the rapid synthesis of hormones. After separating the blood thyroid hormones bind to carrier proteins - thyroxine binding globulin and albumin. The free form is present in the blood of less than 0.5% thyroxine and triiodothyronine.

In the synthesis of thyroid hormones may be involved only pure ( "elementary") iodine. Applicants iodine into the thyroid gland is shaped generally iodide, which is then oxidized and passes into elemental iodine. Subsequently iodine molecule is included in the amino acid tyrosine. Accession to the tyrosine molecule one atom of iodine leads to the formation monoiodotyrosine two atoms - diyodtirozina. These compounds do not yet have the intrinsic properties of hormones the thyroid gland. At the confluence of the two molecules is formed diyodtirozina tetrayodtirozin (thyroxine, T4) - thyroid hormone containing four iodine atoms. If we merge molecules monoiodotyrosine and diyodtirozina formed triyodtirozin (T3).

The thyroid follicles is an accumulation of a specific protein - thyroglobulin. Thyroglobulin is a unique reserve of thyroid hormone. Thyroglobulin is a glycoprotein with a molecular weight of about 600,000 daltons, the size of which is so large that its arrival in whole blood from a thyroid gland is almost impossible. Only in diseases of the thyroid gland, accompanied by destruction of its cells (for example, during the development of thyroiditis - inflammation of the thyroid), thyroglobulin into the bloodstream.

Calcitonin is produced parafollicular cells (C cells) of the thyroid gland, which belong to the diffuse endocrine system. Calcitonin is involved in the regulation of calcium and phosphorus metabolism, increases the activity of osteoblasts - cells that create new bone tissue. Calcitonin Unlike other thyroid hormone is a polypeptide structure. It consists of 32 amino acids.

Regulation of synthesis and release into the blood of thyroid iodine-containing hormones made by the pituitary gland, synthesizing thyroid stimulating hormone (thyrotropin, TSH). TTG enhances the synthesis of hormones T4 and T3 and their allocation in the blood. Another important effect is the increase in TSH thyroid growth. The intensity selection of TSH in the blood is determined by the function of the hypothalamus, synthesizing thyrotropin-releasing hormone (TRH). Thus, the production of thyroid hormone occurs in accordance with the needs of the body and is governed by a complex multi-level mechanism.

The production of thyroid hormones depends on the time of day (the so-called circadian rhythm). Thyrotropin-releasing hormone produced by the hypothalamus in the morning in the greatest concentrations. The concentration of TSH - thyroid stimulating hormone pituitary maximum in the evening and at night. The level of thyroid hormone is greatest in the morning and in the evening is at the minimum value.

There are seasonal variations in the level of TSH and thyroid hormones. The concentration of triiodothyronine increased in winter. At the same time increases the concentration of thyroid-stimulating hormone. The level of T4 - thyroxine significantly during goad does not change. After separation of the blood delivered thyroid hormone in those tissues of the body where their action is necessary (so-called target cell). The target cells of the thyroid hormone T4 "off" one of iodine atom (this process is called deiodination), resulting in a much more active hormone - triiodothyronine, which has a major effect. Receptors for thyroid hormones are present in virtually all human cells.

Synthesis involving regulation of pituitary and hypothalamus ensures the supply optimal amounts of thyroid hormones in the blood. By reducing the production of thyroid hormones in the pituitary gland releases increased amounts of blood TSH, causing the thyroid function is enhanced, and the amount allocated to it increases hormones. If the thyroid gland secretes too much hormone, the pituitary gland begins to produce less TSH, causing the thyroid activity is reduced.

The function of thyroid hormone is to increase protein synthesis, cell respiratory activity enhancing, stimulating bone growth and brain development. Of particular importance are the thyroid hormones in the first 3 months of pregnancy, when under their influence, there is an active formation of the fetal brain cortex. That is why many pregnant women are recommended to determine the level of thyroid hormones and TSH to determine the sufficiency of their synthesis. Long and marked reduction in thyroid hormone levels during pregnancy significantly reduces the IQ of the newborn, as well as increases the risk of pregnancy complications and premature interrupt her. Reduced thyroid function in adults is called hypothyroidism. The main reason for reducing the production of thyroid hormones is the defeat of the thyroid gland cells of the patient's own immune system. As a result of such an autoimmune condition number actively functioning cells is significantly reduced, resulting in a lost opportunity thyroid hormones synthesized in the required amount. Symptoms of a lack of thyroid hormones are the development of edema, dry skin, the active hair loss, reduced body temperature, slow mental activity, the development of depression. By reducing the levels of thyroid hormones decreases blood flow and oxygen consumption of the brain slows the growth and formation of nervous tissue and skeletal ossification. Treatment of this condition is most often carried out taking synthetic thyroid hormones that allow the safe and effective to eliminate the symptoms of hypothyroidism.

Amplification of thyroid function in development occurs when thyroid tissue nodes autonomously functioning producing hormones in excess. At the same time begins to dominate catabolic action of thyroid hormones - increases metabolism, actively burned supplies energy-rich substances (glycogen, fat) that leads to excessive heat generation and increase in body temperature, sweating, as well as increased heart rate (tachycardia). This condition is called hyperthyroidism (thyroid hormone poisoning). When thyrotoxicosis is usually marked decrease in the patient's body weight, however, take drugs thyroid hormones to lose weight should not be - the amount of side effects of such "treatment" will be much more than a positive effect. Determining the level of thyroid hormones is currently being conducted in many laboratories, but the techniques used by laboratories may vary considerably among themselves. The best way to count the number of blood thyroid hormones is immunohemilyuminestsentny (the so-called third-generation method). The probability of errors in the laboratory using the method immunohemilyuminestsentnogo minimal, and the speed of the analysis, on the contrary, is maximal. In this test, the cost remains low.